Some new commercial systems for sterilizing medical instruments and the like utilize low-temperature reactive gas plasma to achieve rapid, low-temperature, low-moisture sterilization of medical items. Low-temperature gas plasma is sometimes described as a reactive cloud which may contain ions, electrons, and/or neutral atomic particles. This state of matter can be produced through the action of electric or magnetic fields, or through other external forces such as high-energy particle flux. In general, an electric field can be in any frequency range (An example of a naturally occurring plasma is the aurora borealis or the northern lights). One commercial embodiment of plasma sterilization is the STERRAD.RTM. Sterilization Process practiced by the assignee of the present application. The STERRAD.RTM. process is performed in the following manner. The items to be sterilized are placed in the sterilization chamber, the chamber is closed, and a vacuum is drawn. An aqueous solution of hydrogen peroxide is injected and vaporized into the chamber so that it surrounds the items to be sterilized. After reduction of the pressure in the sterilization chamber, a low-temperature gas plasma is initiated by applying radio frequency energy to create an electrical field. In the plasma, the hydrogen peroxide vapor is dissociated into reactive species that collide/react with and kill microorganisms. After the activated components react with the organisms or with each other, they lose their high energy and recombine to form oxygen, water, and other nontoxic byproducts. The plasma is maintained for a sufficient time to achieve sterilization and remove residuals. At the completion of the process, the RF energy is turned off, the vacuum is released, and the chamber is returned to atmospheric pressure by the introduction of High Efficiency Particulate-filtered Air (HEPA).
The above-described sterilization system can safely process medical items currently sterilized by ethylene oxide and steam, with the exception of linens, other cellulosic materials, powders, and liquids. Sterilized items are ready to be used in a little over an hour after starting the sterilizer. The process requires no aeration, and there are no toxic residues or emissions. Preparation of instruments for sterilization is similar to current practices: cleaning the instruments, reassembly, and wrapping. The system typically uses non-woven polypropylene wraps, which are commercially available, and a special tray and container system. A special adaptor placed on long, narrow lumen instruments allows rapid sterilization of their channels. A chemical indicator specifically formulated for this process is used, as well as a specifically designed biological indicator test pack.
The efficacy of the STERRAD plasma sterilization system has been demonstrated by: (1) killing a broad spectrum of microorganisms; (2) killing highly resistant bacterial spores in less than one-half of the full sterilization exposure cycle; (3) killing highly resistant bacterial spores on 16 different substrates commonly used in medical items. Depending upon the particular design plasma sterilization systems can therefore provide efficient, safe methods for sterilizing medical instruments and other hospital products.
For optimum operation, a plasma sterilization system such as that described above requires the loads that are to be sterilized to be quite dry. However, normal hospital practice in the preparation of instruments for sterilization often results in levels of water that may be excessive. The excess water makes it difficult to achieve the low-pressure thresholds required to initiate the sterilization process. To initiate the sterilization process, the chamber pressure is preferrably reduced to relatively low levels, for example approximately 200-700 mTorr. Since the equilibrium vapor pressure of water is significantly higher than 700 mTorr at room temperature, any water in the chamber or load will begin to vaporize during the vacuum phase. The heat of vaporization required for the water to vaporize causes the load and any remaining water to chill. When enough water has vaporized, the remaining liquid begins to freeze. Eventually, the remaining liquid will completely freeze, which slows the rate of vapor generation and retards the attainment of the pressure levels required for optimum operation of the sterilizer. These conditions can cause undesirably long sterilization cycles or even cancellation of the sterilization cycle. To avoid this problem, a method is needed for preventing or removing any solid water in the vacuum chamber so that the desired pressure may be quickly achieved for sterilization.
Gaseous ion bombardment of surfaces in vacuo, commonly known as sputtering, is often used to remove adsorbed molecular species from surfaces and even to remove surface layers of the material itself. Although, it is known that noble gas plasma sputtering may enhance outgassing in high and ultra high vacuum systems, the energy and momentum exchange mechanisms between the plasma and surface can also lead to material damage of the surface as well as emission of the adsorbed species. Clearly, sputtering with the attendant material damage is unacceptable for a sterilization process.